With the forming technique generally used for producing cupola-shaped plastic pieces, a totally flat plastic sheet is heated above its softening temperature by a radiant heater, the rim is clamped tight on a forming device, and the area surrounded by the rim is formed. After cooling of the shape, the rim clamping is released, and the shape is then removed from the forming device. See, e.g., the company publication of Imperial Chemical Industries Ltd. entitled "Acrylic Materials, Shaping by Heat Forming" (1960), at pages 12, 13, 64, and 65.
The foregoing technique has the disadvantage that the flat rim of the cooled shape is unevenly warped as a result of internal stresses. Consequently, as a rule the flat rim of the cooled shaped must be cut off. The internal stresses arise from the different conditions in different parts of the shape during the cooling phase. While the unclamped parts can thermally contract during cooling below the softening temperature, the tightly clamped rim area is prevented from doing so. Warping of the rim cannot be prevented by release of the clamping tool soon after the plastic has cooled to below its softening temperature. Even severe warpings occur in the rim area if the clamped sheet is heated in the forming device by radiant heat. However, the use of just such a production method is desirable because it involves little labor input.